Photodissociation of phosgene: Theoretical evidence for the ultrafast and synchronous concerted three-body process

Abstract

The potential energy surfaces for Cl(2)CO dissociation into CO+Cl+Cl in the lowest two electronic singlet states (S(0) and S(1)) have been determined by the complete active space self-consistent field, coupled-cluster method with single and double excitations (CCSD), and equation-of-motion CCSD calculations, which are followed by direct ab initio molecular dynamics simulations to explore its photodissociation dynamics at 230 nm. It is found that the C-O stretching mode is initially excited upon irradiation and the excess internal energies are transferred to the C-Cl symmetric stretching mode within 200 fs. On average, the first and the second C-Cl bonds break completely within subsequent 60 and 100 fs, respectively. Electronic structure and dynamics calculations have thus provided a strong evidence that the photoinitiated dissociation of Cl(2)CO at 230 nm or shorter wavelengths is an ultrafast, adiabatic, and concerted three-body process. Since the two C-Cl bonds begin to break at the same time and the time interval between the two C-Cl bond broken fully is very short ( approximately 40 fs), the photoinitiated dissociation of Cl(2)CO to CO+2Cl can be considered as the synchronous concerted process.

Predissociative T1and S1states of carbonyl chlorofluoride (ClFCO) and their photodissociation pathways

“…lower than the corresponding value (0.265 eV) of the S 1 state of Cl 2 CO [40]. The minimum-energy curves for the C-F bond (Figure 2(c)) have similar character to those in Figure 2(b).…”

“…Thus the observation of the molecular fragment CO in the 193 nm experiment can be explained. The ratio between the excess energy and the C-Cl bond dissociation energy in this case is very close to the corresponding ratio for the photodissociation of Cl 2 CO with 230 nm light [40]. The excess energy of 230 nm light above the 'Cl 2 CO(X, 1 A 0 )!Cl(X 2 P) þ ClCO(X 2 A 0 )' dissociation limit is approximately 2.28 eV, which is approximately three times greater than the C-Cl bond dissociation energy of the ClCO radical.…”

“…According to the geometrical structure of TS S 1 ðCÀFÞ , the C-F bond length is greater by approximately 0.57 Å than that of the stationary S 1 state structure of ClFCO, and this elongation is approximately twice the elongation of the C-Cl bond length at the transition state of S 1 of Cl 2 CS [40]. The excess energy of 0.217 eV in the case of 'ClFCO(X, 1 A 0 )!F(X 2 P) þ ClCO(X 2 A 0 )' dissociation under 191 nm light is slightly smaller than the excess 1434 K. Lee and K.K.…”

“…The excess energy of 230 nm light above the 'Cl 2 CO(X, 1 A 0 )!Cl(X 2 P) þ ClCO(X 2 A 0 )' dissociation limit is approximately 2.28 eV, which is approximately three times greater than the C-Cl bond dissociation energy of the ClCO radical. In a recent theoretical study of Cl 2 CO, Fang et al [40] showed that the actual photodissociation dynamics of the S 1 state of Cl 2 CO corresponds to an almost synchronous three-body dissociation: the time interval between the first C-Cl bond dissociation and the second C-Cl bond dissociation is only approximately 40 fs [40]. Note the similarity between the dissociation of the C-F bond of ClFCO under 193 nm light and the dissociation of the C-Cl bond of Cl 2 CO under 230 nm light: thus synchronous three-body dissociation could be the main source of CO fragments from ClFCO, as in the case of Cl 2 CO.…”

“…As the C-Cl bond length increases, the electronic configuration of the S 1 state changes from n O Ã ðC¼OÞ to n O Ã ðCÀCl Þ , as is the case for the S 1 state of Cl 2 CO [40]. Very small energy barriers (without including the zero-point vibration energy) of only 0.18 and 0.29 eV were calculated for T 1 and S 1 , respectively.…”